Polarization

Discussion

Light is a transverse electromagnetic wave that can be seen by a typical human. Wherever light goes, the electric and magnetic fields are disturbed perpendicular to the direction of propagation. This propagating disturbance is what makes light a wave. The fact that the electric and magnetic fields are disturbed makes light an electromagnetic wave. The fact that it disturbs these fields at right angles to the direction of propagation makes light a transverse wave. In this section we will explore what it means to be transverse.

Imagine a light wave traveling toward you, on its way to entering your eye. In what direction is the electric field vibrating? (Light is both electric and magnetic, but it is usually the electric field that we are interested in.) Up and down? Sure. Left and right? Sure, why not. Both alignments are perpendicular to the propagation of the wave.


Most light sources are unpolarized. The electric field is vibrating in many directions; all perpendicular to the direction of propagation. Polarized light is unique in that it vibrates mostly in one direction. Any direction is possible as long as it's perpendicular to the propagation, be it …

horizontal, vertical, or diagonal.

How to produce polarized light.

  1. reflection from a dielectric surface
    glare
    reflected light is partially polarized at right angles to the plane of incidence
    Brewster angle: reflected light is completely polarized when the reflected ray is perpendicular to the refracted ray (show that the tangent of the Brewster angle equals the index of refraction)
  2. scattering
    blue sky, rainbow
    light scattered from small molecules is polarized at right angles to the direction of propagation of the original beam
  3. dichroic crystals
    Dichroism: The property of presenting different colors by transmitted light, when viewed in two different directions, the colors being unlike in the direction of unlike or unequal axes.
    absorbs the component of wave polarized in a particular direction
    tourmaline
    quinine iodosulfate in viscous plastic — crystals oriented by extrusion
    giant thin crystals of iodosulfate of quinine
    Herapathite: the sulfate of iodoquinine
    Edwin Land (1909-1991) United States
  4. birefringent crystals
    calcite (Iceland spar)
    crystal with a preferential direction of polarized and propagation
    "o" ray and "e" ray (ordinary and extraordinary)

linear vs. circular polarization


[animate]

Polarized light carries information. Magnetic fields, chemical interactions, crystal structures, quality variations, and mechanical stresses can all affect the polarization of a beam of light.

spectroscopy, polarimetry, defectoscopy, astronomy, platography, material research, laser applications, light modulation, agricultural production, electric power generation, environmental control devices, molecular biology, biotechnology

polarized sunglasses, photography

glare, scattering (polarized sky)

liquid crystals

Blah.

3d movies.

stereochemistry

all sugars are "right-handed", but may rotate the polarization of light in either direction: right/clockwise/dextrorotatory, left/counterclockwise/levorotatory.

Determining whether a particular compound is right or left handed is determined by a particularly complicated set of rules that I do not understand (and don't care to understand at this moment).

chirality: right-handed sugars and left-handed proteins (except glycine, which is not chiral) exist naturally, the other isomers may be indigestible, may cause diarrhea, may be toxic, some organic compounds exist in both forms e.g., carvone (a diterpene) has two enantiomers: D-(+)-carvone which is found in the seed oils of caraway, dill, and anise; and L-(-)-carvone which is found in spearmint oil.

Glucose is dextrorotatory, also called dextrose, α-D-(+)-glucose (C6H12O6 six-sided ring)

fructose is levorotatory, also called levulose, α-D-(-)-fructose (C6H12O6 five-sided ring)

All 20 amino acids but one (glycine) are optically active chiral molecules

sucrose is a disaccharide of glucose and fructose, sucrose is dextrorotatory, when the two molecules are separated (hydrolyzed) the rotation caused by the fructose dominates making the mixture levorotatory, thus the polarization of the solution has been "inverted", the sugars themselves have not had their chirality inverted, this would require inversion of the molecule in 3 separate places (unlikely)

"Enantiomers are stereoisomers that are precise mirror images of each other. They demonstrate equal amounts, but opposite directions of optical rotation. In all other respects, their physical and chemical properties are identical. Their physiological actions may differ, because enzymes and other biological receptors can readily discriminate between many enantiomeric pairs."

photoelastic stress

text

Stress patterns appear in transparent materials sandwiched between crossed polarizers. Some manufacturing processes can introduce permanent stress. This protractor will most likely break where the stress lines are most dense (at 60° on the arc or 4 1/4 inches on the base).

follow up?

The Physics Hypertextbook

  • No condition is permanent.